1. Field of the Invention
This invention relates to seismic surveying and more particularly to a method and apparatus for initiating an acoustic wave in a body of water.
2. Description of the Related Art
Oil and gas exploration techniques include seismic surveying on land and at sea. Seismic surveying requires the introduction of energy into the earth. The energy is typically in the form of an acoustic wave. In marine seismic surveys, the acoustic waves penetrate the earth""s crust and are reflected from the various strata therein. These reflected waves are analyzed to provide information indicative of the content and location of the subterranean strata from which the reflections take place. In these marine seismic surveys, the seismic wave is generated by a number of known sources such as an air gun array towed with a seismic vessel.
The term air gun as used herein refers to any number of devices used to initiate an acoustic wave in a body of water. An air gun has a pressurized chamber within a housing and a shuttle for opening and closing a port in the housing. When the port is opened, pressurized fluid such as air or other inert gas contained in the pressurized chamber rapidly egresses the chamber. The escaping fluid creates the seismic wave.
A solenoid valve assembly typically activates the shuttle. When the solenoid valve is activated, a firing chamber in the air gun assembly pressurizes to open the shuttle. Known valve assemblies include inertial solenoid valve assemblies and pressure balanced solenoid valve assemblies. Examples of these valves can be found described in the following U.S. Pat. No. 4,928,785 xe2x80x9cFull Flow Solenoid Valve for Air Gun Operationxe2x80x9d to Harrison, U.S. Pat. No. 3,929,315 xe2x80x9cSolenoid Valve Assemblyxe2x80x9d to Rieth, U.S. Pat. No. 3,800,832 xe2x80x9cAir Gun Firing Assemblyxe2x80x9d to Umphenour et al., and U.S. Pat. No. 5,301,920 xe2x80x9cHigh-Speed Solenoid Valve Apparatus to Ichiki.
In the operation of an air gun using valves such as those identified above, the solenoid valve delivers high-pressure air to a firing chamber to activate a shuttle valve, which opens to allow an explosive egress of air from the gun into a body of water. This operation is commonly known as xe2x80x9cfiringxe2x80x9d the gun even though no detonation or combustion takes place. The shuttle valve opens when the firing chamber reaches a triggering pressure. The rate at which pressure increases within the firing chamber determines the accuracy of triggering time. High pressure rate results in a smaller rise time or xe2x80x9ctime windowxe2x80x9d to trigger the shuttle valve and provides better accuracy of triggering time. Therefore, it is extremely important that the solenoid valve provides quick pressure rise.
Rise time is extremely important in understanding reflected waves. Wave analyses must be referenced to a time at which the acoustic wave was generated to accurately determine distance between the source and reflecting surface. A perfect input signal would be a step function with sharp contrast between pre and post firing of the gun and at a particular point in time. The slower the rise time of a input device, the more approximate the firing time. Therefore any determination based on the rise time becomes less accurate with increasing rise time.
In a typical valve, either a pressure balanced or a spring-loaded plunger is used between inlet and outlet ports. The plunger opens relatively slowly due to the pressure load or spring load. This increases the rise time. Additionally, guides for the moving plunger are consistently located in the flow path restricting airflow from solenoid valve to the firing chamber. Another disadvantage in a typical valve is that the moving parts are not protected from wear and tear caused by friction or impact between internal components. Thus the operational life of the typical valve is limited.
The operational life of a typical valve currently used is on the order of 150,000 to 200,000 shots or activation cycles. Ocean surveys may extend over thousands of miles of criss-crossing grid survey paths or lines. And during these surveys, shuttle activation valves must cycle several thousand times making the valves expensive maintence items in terms of cost and time required to repair a gun with a failed valve.
The inertial valve assembly suffers from short operational life and slow rise time. The operational life is limited by component impact caused by high pressure air forcing the valve closed after activation. The valve activation in a solenoid inertial valve assembly must overcome a high pressure differential to initiate the valve opening, and this causes a slow rise time.
The pressure balanced solenoid valve includes a pressure chamber in the valve to equalize pressure across two plunger seals. The pressure balanced valve reduces the force required to open the plunger. This reduced force tends to reduce wear and tear on the valve components. A drawback of the typical pressure balanced valve is that the plunger must extend into the flow path and restricts air flow to the outlet port of the valve assembly. This restricted flow increases the rise time for pressurizing the firing chamber.
The present invention overcomes some of these drawbacks by providing a pressure-balanced inertial valve assembly for use in seismic surveys.
In one aspect of the present invention a valve assembly is provided for use in applications requiring fast operation coupled with long operational life. Provided is a valve assembly comprising a valve housing having a first sealing surface. A poppet is in the housing, the poppet has a first end adapted for sealing engagement with the first sealing surface. A fluid chamber is within the housing for containing a fluid under pressure. The fluid chamber has a fluid passage that connects the fluid chamber to a second end of the poppet at a second sealing surface disposed between the second end of the poppet and the fluid passage. The fluid in the fluid chamber and the fluid passage exerts a predetermined pressure on each of the first and second sealing surfaces defining a pressure differential. An exit port in the housing is provided for releasing the fluid. An inertial mass is movably coupled to the poppet for moving the poppet from a sealed position to an open position.
In another aspect of the invention a seismic air gun for creating an acoustic wave in a body of water is provided. The air gun comprises a gun housing and a first chamber within the gun housing for containing a first fluid under pressure. A shuttle is operably coupled to the first chamber for opening the first chamber to the body of water. A second chamber is adapted to be pressurized by a second fluid, wherein the shuttle is operated upon pressurizing the second chamber with the second fluid. A solenoid valve assembly supplies the second chamber with the second fluid. The solenoid valve assembly further comprises a valve housing having a first sealing surface, a poppet having a first end adapted for sealing engagement with the first sealing surface, and a valve chamber within the housing for containing the second fluid under pressure. The valve chamber has a fluid passage connecting the valve chamber to a second end of the poppet at a second sealing surface disposed between the second end of the poppet and the fluid passage. The second fluid in the valve chamber and the fluid passage exert a predetermined pressure on each of the first and second sealing surfaces defining a pressure differential. An exit port in the housing is provided for releasing the fluid. A coil assembly is operatively associated with the poppet for moving the poppet axially within the housing, wherein the coil assembly includes a coil and a core movably coupled to the poppet for moving the poppet from a sealed position to an open position when the coil is provided with electrical energy. A controller is provided for controlling the solenoid valve assembly.
In another aspect of the present invention, a method is provided for activating an air gun with a valve assembly. The valve assembly includes a valve housing having a first sealing surface, a poppet having a first end adapted for sealing engagement with the first sealing surface, and a fluid chamber within the housing for containing a fluid under pressure. The fluid chamber has a fluid passage connecting the fluid chamber to a second end of the poppet at a second sealing surface disposed between the second end of the poppet and the fluid passage. The fluid in the fluid chamber and the fluid passage exert a predetermined pressure on each of the first and second sealing surfaces defining a pressure differential. An exit port in the housing is provided for releasing the fluid. An inertial mass is movably coupled to the poppet for moving the poppet from a sealed position to an open position. The method comprises sealing the first surface with the poppet, pressurizing the fluid chamber with the fluid thereby exerting the predetermined pressure on each of the first and second sealing surfaces to define the pressure differential, moving the poppet from a sealed position to an open position using the inertial mass, and releasing the fluid through the exit port in the housing, and activating the air gun with the released fluid.